1.Field of the Invention
The present invention relates to a cascode amplifier made up of silicon CMOS of relatively low manufacturing cost, and more particularly, to a cascode amplifier with an expanded range of supply voltage control and capable of preventing deterioration of power added efficiency.
2.Background Art
Elements using processes of GaAs HBT or the like of good high-frequency characteristics and a high withstand voltage are generally used for high output amplifiers for amplification of transmission signals used for mobile communication devices such as mobile phones. High output amplifiers manufactured through these processes are designed to achieve maximum power added efficiency at maximum output power and the power added efficiency drastically deteriorates when output power decreases. A method of preventing efficiency deterioration when output power decreases is available which controls a supply voltage of a high output amplifier using a DC-DC converter in accordance with output power (e.g., see Douglas A. Teeter, “Average Current Reduction in (W) CDMA Power Amplifiers”, Radio Frequency Integrated Circuits (RFIC) Symposium, 2006).
Manufacturing costs of processes of GaAs HBT or the like are relatively high, and therefore high output amplifiers are also being developed which use silicon CMOS processes with relatively low manufacturing costs in volume production. In the case of a high output amplifier made up of CMOS, elements having high transistor high-frequency characteristics have a low withstand voltage, while elements having a high withstand voltage have lower high-frequency characteristics. Therefore, a high output amplifier is configured as a cascode amplifier, elements with a low withstand voltage and high high-frequency characteristics are used for source-grounded transistors and elements with a high withstand voltage and low high-frequency characteristics are used for gate-grounded transistors. A gate voltage of a gate-grounded transistor is set to a voltage at which a drain voltage of a source-grounded transistor does not exceed a withstand voltage.